1. Field of the Invention
The present invention relates to novel kojic acid derivatives, and more particularly it relates to novel kojic acids substituted at the 2-position with 3,4-dihydroxycinnamic acid or 4-hydroxy-3-methoxycinnamic acid. These derivatives exhibit a strong skin whitening activity.
2. Description of Prior Art
Kojic acid, which is obtained from culturing Aspergillus, strongly inhibits the action of tyrosinase, an enzyme involved in the formation of melanin, which is a major factor in determining the color of human skin. It has been reported that kojic acids inhibit the activity of tyrosinase by forming a chelate with the copper ion in the tyrosinase through the 5-hydroxyl and 4-carbonyl groups. Based on this tyrosinase-inhibiting activity of kojic acid, various cosmetic compositions have been proposed containing kojic acid as an active ingredient(See JP 56-18569B).
Further, JP 54-92632A, JP 56-77272A, JP 60-7961B and JP 60-9722B disclose methods for improving the properties of kojic acid, such as storage stability, compatibility, solubility, and the like, and various kojic acid derivatives such as kojic mono- or di-fatty acid esters, having an improved activity of inhibiting tyrosinase. Moreover, JP 3-14508A, JP 4-145096A, JP 4-187618A and JP 5-39298A propose various kojic acid derivatives having a strong tyrosinase-inhibiting activity, such as kojic ethers, glucosylated kojic acids and amino-protected amino acid kojic acids. Furthermore, JP 62-3820B, JP 64-83008A, JP 1-121205A and JP 2-028105A disclose compositions incorporating various additives to improve the solubility of kojic acid and to enhance the skin-whitening activity.
The present inventors have discovered novel kojic acid derivatives having an improved tyrosinase-inhibiting activity as well as decreased side effects to human skin. They have found certain kojic acid derivatives of which the 2-hydroxymethyl group substituted with mono- or di-hydroxycinnamic acid show a superior tyrosinase inhibiting activity. Particularly, 2-dihydroxy cinnamoyl kojic acid exhibits an excellent activity to scavenge radicals, which are known to cause skin aging, and also exhibits decreased side effects to the skin.